The invention relates to applying a chemical treatment to fibers which are suitable for processing into a composite. More particularly, the invention relates to applying a chemical treatment to fibers where the chemical treatment has a low viscosity and is substantially free of an unreactable solvent. Even more particularly, the invention relates to using heat energy to lower the viscosity and improve the wetting ability of a chemical treatment after being applied to the fibers and/or to increase the molecular weight of or cure the applied chemical treatment with very little, if any, generation of volatile organic carbon (VOC).
The present invention also generally relates to the manufacture of fiber-reinforced composite articles, in particular, to wire-coated fiber/polymer composite strands used in molding fiber-reinforced composite articles. More particularly, the invention relates to thermoplastic-encased fiber/polymer composite threads and pellets moldable into fiber-reinforced thermoplastic composite articles.
Fibers or fibrous materials are often used as reinforcements in composite materials.
Glass and other ceramic fibers are commonly manufactured by supplying the ceramic in molten form to a bushing, drawing fibers from the bushing, applying a chemical treatment, such as a size, to the drawn ceramic fibers and then gathering the sized fibers into a tow or strand. There are basically three known general types of chemical treatmentsxe2x80x94solvent-based systems, melt-based systems and radiation cure-based systems.
In a broad sense, the solvent-based chemical treatments include organic materials that are in aqueous solutions (i.e., dissolved, suspended, or otherwise dispersed in water), as well as those that are dissolved in organic solvents. U.S. Pat. Nos. 5,055,119, 5,034,276 and 3,473,950 disclose examples of such chemical treatments. The solvent (i.e., water, organic solvent, or other suitable solvent) is used to lower the viscosity of the chemical treatment to facilitate wetting of the glass fibers. The solvent is substantially unreactable with the other constituents of the chemical treatment and is driven out of the chemical treatment after the wetting of the glass fibers. In each process for applying solvent-based chemical treatments, an external source of heat or some other device external to the fibers is used to evaporate or otherwise remove the water or other solvent from the applied chemical treatment, leaving a coating of organic material on the glass fibers. One drawback to a solvent-based process is that the added step of removing the solvent increases production costs. In addition, some organic solvents are very flammable in vapor form and pose a fire hazard. Another problem with solvent-based systems is that it is very difficult, if not impossible, to remove all of the solvent from the applied chemical treatment. Therefore, solvent-based chemical treatments are limited, as a practical matter, to those systems where any residual solvent left behind in the coating of organic material remaining on the fibers will not have a significantly adverse affect.
With prior melt-based chemical treatments, thermoplastic-type organic solids are melted and applied to the glass fibers. U.S. Pat. Nos. 4,567,102, 4,537,610, 3,783,001 and 3,473,950 disclose examples of such chemical treatments. One disadvantage of prior melt-based processes is the energy costs associated with melting the chemical treatments. The organic solids used with prior melt-based systems are melted at relatively high temperatures in order for the melted organic solids to be applied to the glass fibers. The high temperatures are needed because the organic solids used in the past have relatively high molecular weights. Such high melt temperatures also pose the risk to workers of being burned by the equipment used to melt the plastic material and by the molten plastic material itself. In addition, specialized equipment is typically needed to apply and otherwise handle the high-temperature molten plastic material.
The radiation cure-based chemical treatments are typically acrylate-based organic chemicals, either with or without a solvent, which are cured with ultraviolet radiation via a photoinitiator. U.S. Pat. Nos. 5,171,634 and 5,011,523 disclose examples of such chemical treatments. A major disadvantage to processes using such chemical treatments is that the radiation used, such as ultraviolet radiation, and the chemical treatment used, such as acrylates, are relatively hazardous, often requiring special handling and safety precautions. Some of these processes, such as that disclosed in U.S. Pat. No. 5,171,634, require the radiation curing to be repeated a number of times to obtain the maximum benefit. Each additional radiation-curing step increases the risks involved and adds additional cost to the process. Furthermore, radiation-curable thermoset plastics, and their requisite photoinitiators, represent a highly specialized area of thermoset chemistry. As a consequence, such radiation-cured chemical treatments are expensive and not generally compatible with various classes of matrix resins.
In order to fabricate composite parts, the strands of glass fibers are often further chemically treated in an off-line impregnation process with a polymeric resin. The resin can be a thermoset, either one- or two-part, or a thermoplastic. In one example, previously formed and sized continuous glass fibers are impregnated with a thermosetting resin and then pulled through a heated pultrusion die to cure the resin and make the composite article, such as ladder rails. In such an off-line process, the continuous glass fibers must be separated in some manner to allow impregnation of the resin between the fibers and then recombined. This requirement almost always results in the use of additional hardware such as spreader bars, impregnation baths, and drying or curing ovens. These types of processes have the disadvantage that they add cost and complexity to the process. In addition, the resultant extra handling of the glass fibers can cause breakage of the individual glass filaments, and thereby a degradation in the properties of the composite article. Therefore, while such off-line processes may be effective, they are time-consuming and inefficient (e.g., requiring additional process steps) and, thus, expensive.
Accordingly, there is a need in the art for a safer, more efficient and more cost-effective process for applying a chemical treatment to glass fibers, where the viscosity of the chemical treatment is low enough to sufficiently wet the glass fibers without the need for a solvent, where the chemical treatment does not require radiation curing and the viscosity of the applied chemical treatment increases with very little, if any, generation of water, volatile organic carbon (VOC) or other solvent vapor, and where the resulting chemically treated glass fibers are suitable for subsequent processing into a composite article. There is also a need for an in-line process for forming a preimpregnated glass composite strand from a plurality of continuously formed glass fibers which are chemically treated in this manner, where the resulting prepreg strand is suitable for subsequent in-line or off-line processing into a composite article.
The use of composites having fiber-reinforced polymeric matrices is widespread. Fiber-reinforced polymeric composite products have been manufactured using a variety of processes and materials. As referred to above, one such process involves impregnating one or more strands or bundles of reinforcing fibers (e.g., glass fibers, synthetic fibers or some other reinforcing fibers) with a thermoplastic material, and using the resulting composite strands to mold a composite article. These composite strands have been used in the form of continuous threads (i.e., long lengths of strand) and discrete pellets (i.e., short lengths of strand). The fibers from the composite strands provide the reinforcement and the thermoplastic material forms at least part of the matrix for the composite article.
It is desirable for each fiber strand to be fully impregnated with the thermoplastic matrix material, that is, for the thermoplastic material essentially to be evenly distributed throughout each bundle of fibers and between the fibers. Because all of the fibers start out surrounded by matrix material, the fully impregnated fiber strands can be molded less expensively and more efficiently and the corresponding composite article can exhibit improved properties. However, it is difficult and time-consuming to fully impregnate fiber strands with typical thermoplastic matrix materials (e.g., engineering thermoplastics). Fully impregnating strands at high throughput rates has been particularly difficult, especially at the throughput rates typically experienced during the production of continuously formed glass reinforcing fibers.
In an effort to fully impregnate continuously formed glass fiber strands, the number of fibers used to form each strand (i.e., fiber density) has been reduced from a typical density of about 2000 fibers/strand to 1200 fibers/strand or less, to reduce the time it takes to impregnate each fiber strand. However, by reducing the number of fibers in each strand being processed at a given time, the production output and cost efficiency of the process can be adversely impacted. In addition, fully impregnating even such lower density strands is still sufficiently time-consuming to prevent even the lower density strands from being fully impregnated and processed at the higher throughput rates typically approached in the production of continuous glass reinforcing fibers.
In an effort to obtain higher throughputs, one prior process only partially impregnates the fiber strand and coats the strand in a uniform layer of thermoplastic matrix material, leaving a central core of fibers not impregnated with the thermoplastic. This coating and partial impregnation of the strand is accomplished by pulling the strand through what has been referred to as a xe2x80x9cwire-coatingxe2x80x9d device. Wire-coating devices, such as that disclosed in U.S. Pat. No. 5,451,355, typically include an extruder for supplying molten thermoplastic matrix material and a die having an entrance orifice, an exit orifice and a coating chamber disposed therebetween. The extruder supplies molten thermoplastic material to the coating chamber. The strand is coated and partially impregnated with the thermoplastic matrix material as it passes through the coating chamber, and the coating is formed into a uniform layer when the coated strand passes through the exit orifice of the die. The resulting coated strand is either used in the form of a thread (e.g., in compression-molding applications) or cut into discrete pellets (e.g., in injection-molding applications). Because the strand is only partially impregnated with the thermoplastic matrix material, the strand can be processed at relatively high throughputs.
However, these partially impregnated wire-coated strands also exhibit a number of problems because of their central core of unimpregnated fibers. When in pellet form, the fibers in the central unimpregnated core tend to fall out of the thermoplastic coating. When the strand is in the form of a thread, the core fibers are less likely to fall out, but the core of these wire-coated threads must still be impregnated at some point to optimize the properties of the resulting composite article. Impregnating the central core of such wire-coated threads during the molding operation can be difficult and time-consuming, if not impossible as a practical matter. Thus, molding with such wire-coated threads can cause a reduction in the overall production rates, rather than an increase as desired.
Therefore, there is a need for a way to produce fully impregnated fiber strands at higher throughput rates, even when each strand has a relatively high fiber density, where the resulting composite strands, either in thread or pellet form, are suitable for molding fiber-reinforced thermoplastic articles.
An object of the invention is to attain a chemical treatment for fibers, such as glass fibers, that is substantially free of unreactable solvent. Another object is to achieve a solvent-free chemical treatment that is substantially non-photosetting. An additional object of the invention is to provide such a chemical treatment that has an enhanced wetting ability. A further object is to provide a solvent-free chemical treatment that may be cured or have its viscosity reduced through application of heat energy to the chemical treatment coated on fibers. Another object of the invention is to provide an advantageous process for applying a chemical treatment to fibers, so that the coated fibers may be made into composite strands useful for forming into composite articles. An additional object is to provide such a process that yields fibers thoroughly impregnated with chemical treatment.
Such objects are achieved via, inter alia, a method of making a composite product, such as a composite strand or a molded article prepared from such a strand product, the method generally comprising preparing a thermoplastic-encased composite strand material for disposing in a matrix material. The thermoplastic-encased composite is prepared by steps comprising: applying a chemical treatment in an amount sufficient to coat substantially all of a plurality of fibers comprising reinforcing fibers to form preimpregnated fibers, wherein the chemical treatment is compatible with the matrix material; gathering the preimpregnated fibers into a preimpregnated strand having the chemical treatment disposed between substantially all of the plurality of fibers; and encasing the preimpregnated strand by a process including wire-coating the preimpregnated strand with a thermoplastic material to form a thermoplastic coating and forming the thermoplastic coating into a thermoplastic sheath to form a thermoplastic-encased composite strand. In a preferred embodiment, the thermoplastic-encased composite strand is cut into lengths to form a plurality of pellets. Alternatively, the thermoplastic-encased composite strand may be packaged as a thread. In one embodiment, the reinforcing fibers include preformed reinforcing fibers. The plurality of fibers may also comprise matrix fibers. The method may also further comprise steps such as preparing the reinforcing fibers by a process including continuously forming reinforcement fibers from molten glass or preforming matrix fibers from a polymeric material. Optionally, the method may comprise preparing the reinforcing fibers in-line by a process including continuously forming reinforcement fibers from a molten glass material. The chemical treatment used in such a method may comprise water and an organic material in an amount providing the preimpregnated strand with an organic material content of from about 2% to about 25% by weight, with substantially all of the water in the chemical treatment being evaporated before the gathering step. The organic material may be a solid or a liquid dispersed or emulsified in the water. More preferably, the organic material content is from about 2% to about 15% by weight, and the evaporating step comprises heating the chemical treatment after the applying step, and even more preferably the organic material content is from about 6% to about 7% by weight, and the heating comprises supplying heat energy to the chemical treatment from an external source or from the plurality of fibers. In one embodiment, the chemical treatment is thermosetting, and the preparing of the thermoplastic-encased composite strand material further comprises the step of at least partially curing the chemical treatment after the applying step. The chemical treatment is preferably substantially solvent-free and substantially non-photosetting, and the organic material comprises a film former and a coupling agent. In one embodiment, the chemical treatment is thermoplastic, the film former includes a low molecular weight thermoplastic polymer, and the coupling agent includes a functionalized organic substrate. In another embodiment, the chemical treatment is thermosetting, the film former includes at least one of a multi-functional monomer and a low molecular weight mono-functional monomer, and the coupling agent includes a functionalized organic substrate. The method may further comprise combining the thermoplastic-encased composite strand with the matrix material to form a composite formulation, and molding the composite formulation. Furthermore, the method may comprise forming the thermoplastic-encased composite strand into pellets, and molding the pellets combined with a resinous matrix material to form a fiber-reinforced composite article. The invention is also directed to products made according to such methods.
Additionally, the invention relates to a composite product comprising a plurality of thermoplastic-encased composite strands useful in forming a fiber-reinforced composite article containing a matrix material, each thermoplastic-encased composite strand comprising a preimpregnated strand comprising a plurality of gathered fibers including reinforcing fibers substantially coated with a thermoplastic or thermosetting chemical treatment compatible with the matrix material. In one embodiment, the composite product comprises pellets cut from the composite strands, with the chemical treatment keeping the plurality of gathered fibers together in the pellets. Alternatively, the composite strands may be packaged in thread form. Preferably, the plurality of gathered fibers numbers in the range of from about 1,500 to about 10,000, more preferably, from about 2,000 to about 4,000. The plurality of gathered fibers may optionally include matrix fibers made from a thermoplastic material. In one embodiment, the chemical treatment comprises an organic material, and each preimpregnated strand has an organic material content of from about 2% to about 25% by weight, more preferably, from about 2% to about 15% by weight, and even more preferably, from about 6% to about 7% by weight. The chemical treatment may be thermoplastic, substantially solvent-free, and substantially non-photosetting, and comprise a (i) film former containing a low molecular weight thermoplastic polymer material and (ii) a coupling agent containing a functionalized organic substrate. Alternatively, the chemical treatment may be thermosetting, substantially solvent-free, and substantially non-photosetting, and comprise (i) a film former containing at least one of a multi-functional monomer and a low molecular weight mono-functional monomer and (ii) a coupling agent containing a functionalized organic substrate. The plurality of composite strands may be molded with a matrix material.
The invention further relates to a method for preparing a composite product, the method comprising the steps of: applying a thermosetting or thermoplastic chemical treatment to a plurality of fibers including glass or synthetic reinforcing fibers to form fibers coated with applied chemical treatment, the chemical treatment being substantially solvent-free and substantially non-photosetting; and heating the applied chemical treatment so as to lower the viscosity of at least a portion of the applied chemical treatment or cure at least partially the applied chemical treatment, or both, to form coated fibers. The chemical treatment may be applied in an amount of from about 0.1% to about 1% by weight to size the plurality of fibers, or in an amount of from about 2% to about 25% by weight to preimpregnate the plurality of fibers. The fibers may further include polymeric matrix fibers. In one preferred embodiment, the reinforcing fibers include glass reinforcing fibers and the heating step comprises supplying heat energy to the applied chemical treatment emanating from the glass reinforcing fibers, with the glass reinforcing fibers being at a temperature preferably of from about 150xc2x0 C. to about 350xc2x0 C., more preferably of from about 200xc2x0 C. to about 300xc2x0 C., during the applying step. The reinforcing fibers may include preformed reinforcing fibers, with the method further comprising the step of pre-heating the preformed reinforcing fibers. Also, the reinforcing fibers may include glass fibers, with the method further comprising the step of forming the glass fibers from a source of molten glass reinforcing material, where the heating step includes supplying heat energy retained in the glass reinforcing fibers from the forming step to the applied chemical treatment. The heating step may include supplying to the applied chemical treatment heat energy from a source external to the plurality of fibers. In one preferred embodiment, the chemical treatment is thermosetting and the heating step cures at least partially a portion of the applied chemical treatment. Alternatively, the chemical treatment is thermoplastic and the heating step lowers the viscosity of at least a portion of the applied chemical treatment. The method may further comprise a step of gathering the coated fibers together into a composite strand, and the heating step may occur after the gathering step. The chemical treatment may contain an organic material, with the composite strand having an organic material content of from about 2% to about 25% by weight. The method may also include the step of forming the composite strand into a composite article having the plurality of fibers disposed in a matrix formed at least in part by the applied chemical treatment. The plurality of fibers optionally includes polymeric matrix fibers forming at least part of the matrix of the composite article. The forming step may be performed in-line with the gathering step. Additionally, the reinforcing fibers and matrix fibers may be commingled to provide the plurality of fibers. The applying step may involve simultaneously coating the reinforcing fibers and the matrix fibers with the chemical treatment.
Additionally, the invention relates to apparatus for carrying out the above methods.
The invention also relates to a chemical treatment for applying to fibers for processing into a composite strand useful for disposing in a matrix material to form a fiber-reinforced composite article, the chemical treatment comprising: a film former comprising at least one of a multi-functional monomer and a low molecular weight mono-functional monomer; and a coupling agent comprising a functionalized organic substrate. The chemical treatment is thermosetting, at least partially heat curable, substantially solvent-free, and substantially non-photosetting. Optionally, the treatment may include a processing aid, e.g., an epoxy-functional viscosity modifier or butoxyethylstearate. In a preferred embodiment, the chemical treatment is heat-curable at a temperature of from about 150xc2x0 C. to about 350xc2x0 C. The film former may comprise a monomer selected from polyester alkyds, epoxy resins, and compounds containing glycidyl ether functional groups. The film former may also comprise at least one member selected from urethanes, vinyl esters, amic acid, Diels Alder reactive species, and Cope-rearranging compounds. Preferably, the chemical treatment has a viscosity of up to about 300 centipoise (cps) at a temperature in the range of from about 93xc2x0 C. to about 110xc2x0 C.
Furthermore, the invention relates to a chemical treatment for applying to fibers for processing into a composite strand useful for disposing in a matrix material to form a fiber-reinforced composite article, the chemical treatment comprising a film former comprising at least one low molecular weight thermoplastic polymer material and a coupling agent comprising a functionalized organic substrate, wherein the chemical treatment is thermoplastic, substantially solvent-free, and substantially non-photosetting. Optionally, the treatment may comprise a processing aid. The low molecular weight thermoplastic polymer may include a cracked polyester or polyamide, with the polyester or polyamide preferably selected from polyethylene terephthalate, polybutylene terephthalate, and nylon. In a preferred embodiment, the treatment comprises a processing aid including a monomer equivalent selected from di-n-butyl terephthalate, dibenzoate ester of 1,4-butanediol, diethyl terephthalate, dibenzoate ester of ethylene glycol, caprolactone, adduct of adipoylchloride and n-aminohexane, and adduct of 1,6-hexanediamine and hexanoylchloride. Preferably, the chemical treatment has a viscosity of up to about 300 cps at a temperature in the range of from about 93xc2x0 C. to about 110xc2x0 C.
Other objects, features, and advantages of the various aspects of the present invention will become apparent from the detailed description of the invention and its preferred embodiments in conjunction with the appended drawings.